Network Rehabilitation

Network rehabilitation (both mains and service pipes) will reduce the rate at which leaks break out on the network. This will reduce leakage, as well as reducing costs associated with inspections and active leakage control activity highlighted above. Figure 9.5 shows a typical burst frequency distribution curve. This shows that there is a distribution of the frequency at which pipes burst on the network. A small proportion will burst

Burst Rate Distribution

Burst Rate Distribution

Cumulative % of network

Figure 9.5 Burst frequency distribution. (Source: Dave Pearson—Sofia Water.)

at a high frequency, whilst other parts of the network will burst at a much lower frequency. In order to have the greatest impact on leakage one would try to identify those pipes with a high frequency of failure and replace these first. The benefit of replacing further sections of pipe will then be less. Again the law of diminishing return applies, and a point will be reached when it is not economic to replace pipes. A similar curve will exist for the distribution of service pipe bursts across the network.

It has been suggested that there will also be a distribution of background leakage, which will not necessarily be the same as that for burst frequency. Those mains with high burst frequencies may have a low background leakage level and vice versa. This is because background leakage is primarily driven by leakage at joints on service pipes rather than mains themselves. Therefore, network rehabilitation should be targeted at burst and background leakage separately.

To find the economic point, the following calculations are performed:

• The benefit of replacing a section or group of essentially similar pipes in the same locality in terms of reduction in burst frequency and/or background leakage is assessed.

• The cost of replacing these pipes is estimated.

• The reduction in leakage is estimated using component loss modelling.

• The savings in costs in inspections, repairs, and active leakage control are assessed.

• The marginal cost/benefit is assessed as the cost less the sum of the savings divided by the leakage saving.

All the schemes with a cost benefit lower than the cost of water would be deployed. This will establish the economic level of network rehabilitation and the associated leakage level.

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